Battery packs such as lithium-ion secondary batteries are installed in small electronic appliances such as digital still cameras, digital video cameras, mobile phones, or the like, in an attachable/detachable manner.
These battery packs generally have every corner portion in a rounded shape, and, even when a user inserts them in battery housings of electronic appliances mistaking the insertion direction, they will usually not break the inner walls or the like of the battery housings.
Furthermore, among the battery packs, there are some that are made into a horizontally or vertically asymmetric shape by being provided with protrusions such as ribs on the surface so that a user will not insert them in battery housings mistaking the insertion direction of the battery packs. However, battery packs provided with protrusions such as ribs are provided with rough surfaces, which will prevent further miniaturization of the battery packs.